A recently-developed organ culture system for maintaining intact, beating hearts of fetal mice in vitro for several days will be used to study the long-term myocardial effects of a variety of interventions. The influence of certain hormones, substrates, and pharmacological and toxicological agents will be analyzed. Observations will be made on the duration of survival in vitro, rate and strength of beating, histological appearance, metabolism of fatty acids, glucose and amino acids, and lysosomal enzyme activity.